1. Field of the Invention
This invention relates generally to measurement and data acquisition systems and, more particularly, to the design of digital source-measure units.
2. Description of the Related Art
Scientists and engineers often use measurement systems to perform a variety of functions, including measurement of a physical phenomena, a unit under test (UUT), or device under test (DUT), test and analysis of physical phenomena, process monitoring and control, control of mechanical or electrical machinery, data logging, laboratory research, and analytical chemistry, to name a few examples.
A typical measurement system comprises a computer system, which commonly features a measurement device, or measurement hardware. The measurement device may be a computer-based instrument, a data acquisition device or board, a programmable logic device (PLD), an actuator, or other type of device for acquiring or generating data. The measurement device may be a card or board plugged into one of the I/O slots of the computer system, or a card or board plugged into a chassis, or an external device. For example, in a common measurement system configuration, the measurement hardware is coupled to the computer system through a PCI bus, PXI (PCI extensions for Instrumentation) bus, a GPIB (General-Purpose Interface Bus), a VXI (VME extensions for Instrumentation) bus, a serial port, parallel port, or Ethernet port of the computer system. Optionally, the measurement system includes signal-conditioning devices, which receive field signals and condition the signals to be acquired.
A measurement system may typically include transducers, sensors, or other detecting means for providing “field” electrical signals representing a process, physical phenomena, equipment being monitored or measured, etc. The field signals are provided to the measurement hardware. In addition, a measurement system may also typically include actuators for generating output signals for stimulating a DUT.
Measurement systems, which may also be generally referred to as data acquisition systems, may include the process of converting a physical phenomenon (such as temperature or pressure) into an electrical signal and measuring the signal in order to extract information. PC-based measurement and data acquisition (DAQ) systems and plug-in boards are used in a wide range of applications in the laboratory, in the field, and on the manufacturing plant floor, among others. Typically, in a measurement or data acquisition process, analog signals are received by a digitizer, which may reside in a DAQ device or instrumentation device. The analog signals may be received from a sensor, converted to digital data (possibly after being conditioned) by an Analog-to-Digital Converter (ADC), and transmitted to a computer system for storage and/or analysis. Then, the computer system may generate digital signals that are provided to one or more digital to analog converters (DACs) in the DAQ device. The DACs may convert the digital signal to an output analog signal that is used, e.g., to stimulate a DUT.
Multifunction DAQ devices typically include digital I/O capabilities in addition to the analog capabilities described above. Digital I/O applications may include monitoring and control applications, video testing, chip verification, and pattern recognition, among others. DAQ devices may include one or more general-purpose, bidirectional digital I/O lines to transmit and received digital signals to implement one or more digital I/O applications. DAQ devices may also include a Source-Measure Unit (SMU), which may apply a voltage to a DUT and measure the resulting current, or may apply a current to the DUT and measure the resulting voltage. SMUs are typically configured to operate according to what is commonly referred to as “compliance limits”, to limit the output current when sourcing voltage, and limit the output voltage when sourcing current. In other words, a compliance limit on the measured signal may determine the (maximum) value of the sourced signal. For example, when applying a source voltage to a DUT and measuring current, a given current value (e.g. 1 A) specified as the compliance limit would determine the (maximum) input (source) voltage that might be provided to the DUT. In most cases compliance limits may depend and/or may be determined based on the DUTs, e.g. the maximum (absolute) value of the current that may flow into the DUT, or the maximum (absolute) value of the voltage that may be applied across the terminals of the DUT.
In the case of most SMUs, the setpoint (the desired output voltage when sourcing and regulating voltage, or the desired current value when sourcing and regulating current) and the compliance limits are typically programmable. SMUs are available to cover a variety of signal levels, from the microvolt (μV) range to the kilovolt (kV) range, and from the femtoampere (fA) range to the ampere (A) range. Some SMUs can deliver or dissipate significant power, while other SMUs may be operated at low power. The accuracy of SMUs is typically less than the accuracy of high-quality calibrators and/or digital multi meters (DMMs).
For quite a while, SMUs were implemented with precision digital-to-analog converters (DACs) used for programming the setpoint and compliance limits into an analog control loop. The output voltage across the output terminals of the SMU, or the output current flowing into the output terminal of the SMU were typically set using the analog control loops by comparing the outputs to the levels set by the DACs. Each output voltage or output current could be controlled separately, with only one of the analog control loops closed at any given time. Voltage values representative of current and voltage measurements were provided to an analog-to-digital (ADC) element. In some SMUs, separate ADCs (instead of a single ADC) were used to read the analog output voltage or the analog output current. These SMUs were generally limited in flexibility and high in complexity, resulting from requirements to minimize glitches during range switching. In order for the SMU to operate accurately, a high level of accuracy was required for the DACs and ADCs configured in the SMU.
A more recent trend has been to implement SMUs with a control loop configured in the digital domain. The output voltage and output current in such a configuration is measured with dedicated ADCs. When sourcing current, the current readings obtained by the ADCs are compared to a Current Setpoint, and when sourcing voltage, the voltage readings are compared to a Voltage Setpoint, to regulate the current and voltage outputs, respectively. The setpoints can be set, for example, in an FPGA (field programmable gate array) or DSP (digital signal processing) chip. The FPGA or DSP chip can be used accordingly to produce an output to drive a DAC until the output voltage and/or output current reach the respective desired levels. The SMU can be configured to source one type of signal while measuring another type of signal. For example, the SMU can be configured to measure the voltage across the terminals of a device under test (DUT), when sourcing (and regulating) a current to the DUT, and similarly, the SMU can be configured to measure the current flowing into the DUT, when sourcing (and regulating) the voltage applied across the terminals of the DUT.
In addition to controlling voltage and/or current, SMUs are also used to control a combination or interaction of both, for example resistance or power. In particular, the simulation of resistance is useful in various applications. Positive resistance can be used to more accurately simulate the behavior of non-ideal power supplies, which are actually used to provide power to DUTs in real-life operation. Any power supply can be modeled as either a Thévenin or Norton equivalent. FIG. 2 shows the Thévenin equivalent circuit of a non-ideal power supply, modeled with an ideal voltage source 152 connected in series with a resistance 154 of value ‘R’. FIG. 3 shows the Norton equivalent circuit of a non-ideal power supply, modeled with an ideal current source 202 connected in parallel with a resistance 204 of value ‘R’.
An ideal power supply will have a Thévenin equivalent resistance of 0Ω, that is, the value of resistor 154 for an ideal power supply is considered 0Ω. Similarly, an ideal power supply will have an infinite Norton equivalent resistance, that is, the value of resistor 204 for an ideal power supply is considered to be infinite. At low frequencies, SMUs in general attempt to simulate an ideal source as long as they are within their compliance limits. This means that pulling more current when regulating for voltage will not reduce the voltage seen by the DUT. However, it is not always desirable to simulate an ideal power supply, as real-life power supplies are not ideal. Batteries in particular can have fairly high output impedances. Thus, it is desirable to have the capability to programmatically configure the value of R seen by a DUT for either of the equivalent circuits.
Alternatively, as seen in FIG. 2, if the value of the voltage provided by voltage source 152 were 0V (representative of a closed circuit), or, as seen in FIG. 3, if the value of the current provided by current source 202 were 0 A (representative of an open circuit), then the DUT would see a “pure” DC resistance. The simulation of resistance is useful in various applications, most notably when simulating sensors that have a varying resistance with respect to some physical characteristic (such as resistance thermometers or photoresistors). Thus, a system that accurately and programmatically simulates a given resistance can be used, for instance, to test devices that interact with these sensors.
Negative Resistance, though conceptually strange, can also be useful to simulate. Negative resistance can be used to compensate for resistive effects existing in the system. Thévenin negative resistance can be used to negate the resistive effects of cabling or of the DUT load board, in cases where it is impractical to use remote sensing (4-wire) and the test system is sufficiently characterized. Norton negative resistance can be used to negate leakage effects in the test system, in cases where it is impractical to use guarding and special cabling and the system is sufficiently characterized. As an example, FIG. 4 shows a Thévenin equivalent supply 426 with a resistance 422, with a DUT 428 coupled to the terminals of the supply. In this instance, resistances 424 and 430 represent the resistance of the cables/wires coupling DUT 428 to the ideal voltage source 426. Thus, simulating resistance 422 as a negative resistance (−R) having an absolute value commensurate with the total resistance presented by cable resistances 424 and 430 is useful to eliminate the non-ideality of the test fixture represented by those cable resistances.
Present day SMUs feature some resistance simulation, most commonly of the Thévenin variety. In particular, one of National Instruments' own products, the NI PXIe-4154 can configure its output resistance from −0.04Ω to 1Ω, with close to 10-bit resolution (˜1 mΩ). Battery simulators available from various companies have similar ranges and resolutions, though implemented entirely in the analog domain. Currently, “pure” resistance simulation is typically implemented as “Active” resistance simulation. There are presently no combinations of a Norton and Thévenin resistance, with a cut-off point. Some systems, for example, use a PID (Proportional Integral Derivative) controller to directly control the resistance (the setpoint is R, which is compared with V/I).
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.